1. Technical Field
The present invention relates to an A-D converter and an A-D convert method, and particularly relates to an A-D converter outputting a digital output signal, which is a digitalized analogue input signal, and an A-D convert method.
2. Related Art
An A-D converter converts an analogue signal into a digital signal. A-D converters are separated into two types, a single-bit type in which an analogue signal is quantized by a unit of one bit in one clock cycle and a multi-bit type in which an analogue signal is quantized by a unit of plural bits in one clock cycle. As an example of a single bit format A-D converter, a successive approximation type A-D converter (see, e.g., Ricardo E. Suarez, Paul R. Gray and David A. Hodges, “An All-MOS Charge-Redistribution A/D Conversion Technique”, IEEE International Solid-State Circuits Conference, 1974, P. 194-195,248, James McCreary and Paul R. Gray, “A High-Speed, All-MOS Successive-Approximation Weighted Capacitor A/D Conversion Technique”, IEEE International Solid-State Circuits Conference, 1975, P. 38-39,211, JAMES L. McCREARY and PAUL R. GRAY, “All-MOS Charge Redistribution Analog-to-Digital Conversion Techniques—Part 1”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL.SC-10, NO. 6, December 1975, P. 371-379) and a delta sigma type A-D converter are known. As a multi-bit A-D converter, a flash A-D converter, for example, is known.
The multi-bit A-D converter has a relatively short conversion time compared to the single bit A-D converter. However, in a case where the multi-bit A-D converter has high resolution, circuit size thereof becomes large. On the other hand, the circuit size of the single bit A-D converter is relatively small compared to that of the multi-bit A-D converter. However, in a case where the single bit A-D converter has a certain resolution, the conversion time becomes long because one bit at a time is converted.
Furthermore, in a case where high resolution is realized, both the multi-bit A-D converter and the single bit A-D converter experience decreased precision because a quantization range is narrowed, thereby causing a decrease in an acceptable amount of signal noise. In a case where the input signal is amplified by an operational amplifier in an attempt to solve the aforementioned problem, the multi-bit A-D converter and the single bit A-D converter consume an increased amount of power, and furthermore, the precision relies on characteristics of the operational amplifier.